Dentin and enamel

Abstract

The permanent adult human dentition normally consists of 32 teeth, of which 16 are located in the mandible and 16 in the maxilla. There are 4 incisors, 2 canines, 4 premolars and 6 molars for the upper and lower dentition. The incisors are used for cutting food, the canines for tearing, the premolars for grasping, and the molars for grinding (i.e., masticating). There is a generic heterogeneous structure for these teeth, where enamel forms an exterior layer over the underlying dentin. From the cervix to the apex of the root, the exterior of the dentin is covered by cementum to which the periodontal ligament attaches the tooth to alveolar bone. Dental enamel is dense, highly mineralized, hard, and brittle. It contains prism-like structures that span from the enamel surface to the junction of enamel and dentin, the dentino-enamel junction (DEJ). The prisms are comprised of hydroxyapatite crystallites and contain very little organic matrix. These properties make dental enamel an excellent material for cutting and masticating food (i.e., processes that involve friction and wear). In contrast, dentin is not as hard as enamel, but it is tougher. Dentin is a heterogeneous material and can be thought of as a composite structure containing four major components: dentin matrix; dentinal tubules; mineral (i.e., carbonate containing hydroxyapatite); and, dentinal fluid. The dentinal tubules (∼45 000 per mm2) are formed during development of the dentin matrix and are distributed throughout the dentin matrix in a somewhat uniform manner. The dentin matrix mineralizes in an anisotropic fashion, where a highly mineralized tissue, peritubular dentin, surrounds the dentinal tubules. The mineralized tissue between the dentinal tubules and peritubular dentin is referred to as intertubular dentin. Histological examination has revealed that intertubular dentin is less mineralized than peritubular dentin. Furthermore, the matrix and mineral content of root dentin is different from coronal dentin. A good review of the structure of teeth can be found in Waters [1].